In May 1975, Michel Foucault watched Venus rise over Zabriskie Point while Karlheinz Stockhausen’s Gesang der Jünglinge (Song of the Youths) blared from the speakers of a nearby tape recorder. Just a few hours earlier he had ingested LSD for the first time and was in the process of undergoing what he saw as “one of the most important experiences” of his life. And he wasn’t alone. Two newly acquired companions had brought Foucault to Death Valley for this carefully choreographed trip complete with a soundtrack, some marijuana to jump start the effects, and cold drinks to combat the dry mouth. It was all spurred on by the hope that Foucault’s visit to “the Valley of Death”, as he called it, would elicit “gnomic utterances of such power that he would unleash a veritable revolution in consciousness”.
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See Stuart Elden’s comments on his blog in relation to chronology here:

The piece generously quotes my work in Foucault’s Last Decade. But there is one key problem to the idea that this experience changed the course of Foucault’s History of Sexuality – these events occurred in May 1975, 15 months before Foucault finished the first volume. There is a story about how the series changed, which I’ve tried to tell, and which can doubtless be told in other ways, but chronology remains significant.